Thyristors and other semi-conductor power components such as diodes, saturation-undergoing transistors and triacs, are usually dimensioned so as to be able, without the risk of being damaged, to withstand the maximum power they may happen to undergo momentarily. Accordingly, they are fairly oversized with respect to their normal operative power.
In some applications in which currents the value of which may be several times that of the normal operative current (e.g., start-up control, static or hybrid contactors) momentarily flow through the component, it might be an advantage, in order to prevent any oversizing of said component, to be able to obtain an accurate and continuous simulation of the junction temperature-rise, so as to actuate a safety member or an alarm signal whenever a reference temperature that is lower than the maximum temperature permissible is exceeded.
This problem has not yet been solved in a satisfactory manner.
In a semi-conductor power component comprising a housing and a radiator, it is possible, between the junction and the housing bottom, to determine a thermal resistance R.sub.jb, the value of which is given by the manufacturer. In the same way, it is possible to determine a thermal resistance R.sub.br between the housing and the radiator, the value of which can be provided by the manufacturer and a thermal resistance R.sub.ra between said radiator and the environment. The latter resistance, as well as R.sub.br when its value is not provided by the manufacturer, can be measured by sticking a thermocouple on the elements involved and applying the following formula: .DELTA.T=P.times.R.sub.th, R.sub.th being the thermal resistance to be measured, P the transmitted power and .DELTA.T the temperature difference between the element's extremities.
Such a measurement, in practice, cannot be made at the scale of a commercial manufacture, since it is difficult to mount the thermocouple in a stable manner unless an important mass is added, which considerably increases the thermal time-constant of the element measured.
It is however possible to carry out such a measurement in a laboratory on a sample of the component.